Infrared avalanche photodetectors are already known to the person skilled in the art. For example, the known avalanche photodetector element comprises an absorbing intrinsic semiconducting material with bandgap smaller than photon energy of the to-be-detected light wave, including Group IV semiconductors such as Germanium (Ge), Silicon-Germanium (SiGe), Germanium Tin (GeSn) or SiGeSn alloys, or alternatively, binary, ternary and quaternary III-V semiconductors, including but not limited to InP, InGaAs, InGaAsP and related materials.
Light with infrared wavelengths can be coupled directly from free space into the absorbing semiconductor, or alternatively, can be guided and coupled into the absorbing semiconductor by means of an optically transparent input waveguide, which can be built from optically transparent semiconductors such as silicon. The absorbing semiconducting material comprises a p-doped region, an n-doped region, and an intrinsic region in between the doped regions. A schematic overview of such configuration is for example shown in FIG. 1. The p-doped, the n-doped and the intrinsic region are implemented longitudinally along the propagation direction of the incoming light wave and extend with their longitudinal direction parallel to each other and parallel to the longitudinal direction of the absorbing material. The doped regions and the intrinsic semiconducting material thus form a PIN-junction photodiode wherein the input waveguide, if present, and the intrinsic semiconducting material are arranged with respect to each other such that optical waves, with typical infrared wavelengths greater than 1200 nm, guided by the input waveguide are substantially coupled into the intrinsic semiconducting material to the so called PIN-junction, formed by the combination of a p-doped region, an n-doped region and the intrinsic semiconducting material in between them, forming a PIN photodiode.
A reverse bias potential difference is applied between the p-doped region and the n-doped region, resulting in an electric field in the intrinsic region. At the intrinsic, absorbing region of the PIN-junction, the optical signal is converted to an electrical signal by optical excitation of electrons from the valence band to the conduction band under influence of the optical waves, essentially forming free electrons and free holes in the semiconducting material, which are subsequently collected at the n-doped region and p-doped region respectively, under the influence of the internal electric field.
For a sufficiently high electric field in the intrinsic region, the generated free electrons or the generated free holes, or both, can be multiplied by the mechanism known as impact ionization, essentially creating an avalanche of free electrons and/or holes. Such resulting multiplication gain essentially improves the responsiveness of the photodiode, potentially resulting in superior sensitivity of optical receivers built from such avalanche photodetectors. However, it has been found that for creating a sufficiently large multiplication gain of the free electrons and or holes, necessary for increased receiver sensitivity, a relatively strong electric field must be created in between the doped regions. Although the strength of the electric field can be increased by increasing the reverse bias potential difference over the doped regions, the increased potential difference requires for example more power leading to an increase in the power needed to use such avalanche photodetector element.